1. Field of the Invention
The present general inventive concept relates to an illumination system designed to eliminate speckle produced by laser light and a projection system employing the same, and more particularly, to an illumination system designed to efficiently reduce or eliminate laser speckle using a movable diffractive optical element to temporally and spatially average partial beams and a single-panel projection system employing the same.
2. Description of the Related Art
A projection system that uses a laser light source provides a wide range of color rendition due to a wide color gamut while achieving high optical efficiency due to superior collimation. However, the projection system that uses the laser light source suffers from speckle caused by coherency of a laser beam. The speckle results from interference of beams having a random phase when coherent beams are reflected from a rough surface. The speckle degrades a resolution and a quality of an image produced by the projection system.
FIG. 1 illustrates a conventional illumination system disclosed in U.S. Pat. No. 6,606,173 B2 that attempts to eliminate laser speckle. Referring to FIG. 1, the conventional illumination system includes a laser light source 1, a beam expander 2, a holographic diffuser 3, a first field lens 4, and a second field lens 5. All elements of the conventional illumination system are coaxially aligned to convert a Gaussian laser beam emitted by the laser light source 1 into a plurality of uniform beams having rectangular cross-sections.
FIG. 2 illustrates beams having rectangular profiles created by the holographic diffuser 3 in the conventional illumination system of FIG. 1. Referring to FIGS. 1 and 2, a beam emitted by the laser light source 1 is converted to a parallel expanded beam having an increased diameter that is then partitioned into a plurality of rectangular beams by the holographic diffuser 3. Each rectangular beam diverges with horizontal and vertical angles θhoriz and θver, respectively, that can be controlled by a diffraction pattern of the holographic diffuser 3. The first field lens 4 superimposes the plurality of rectangular beams in order to convert the beam having a Gaussian distribution (i.e., the Gaussian laser beam) into a beam having a uniform intensity and to adjust a small rectangular profile of the uniform intensity beam to match an aspect ratio of a micro-display. The second field lens 5 converts the beam that passes through the first field lens 4 to be telecentric.
The holographic diffuser 3 produces rectangular beams with uniform intensity distributions that are then superimposed on one another, thereby eliminating speckle. However, using only the holographic diffuser 3 degrades efficiency of speckle removal and image quality.